The present invention relates in general to substrate manufacturing technologies and in particular to an apparatus for an optimized plasma chamber electrode assembly.
In the processing of a substrate, e.g., a semiconductor substrate or a glass panel such as one used in flat panel display manufacturing, plasma is often employed. As part of the processing of a substrate for example, the substrate is divided into a plurality of dies, or rectangular areas, each of which will become an integrated circuit. The substrate is then processed in a series of steps in which materials are selectively removed (etching) and deposited. Control of the transistor gate critical dimension (CD) on the order of a few nanometers is a top priority, as each nanometer deviation from the target gate length may translate directly into the operational speed and or operability of these devices.
In an exemplary plasma process, a substrate is coated with a thin film of hardened emulsion (such as a photoresist mask) prior to etching. Areas of the hardened emulsion are then selectively removed, causing parts of the underlying layer to become exposed. The substrate is then placed in a plasma processing chamber on a substrate support structure called a chuck (e.g., mono-polar, bi-polar electrode, mechanical, etc.). An appropriate set of plasma gases is then flowed into the chamber and struck to form a plasma to etch exposed areas of the substrate with a particular topography.
Referring now to FIG. 1, a simplified diagram of a capacitive coupled plasma processing system is shown. In general, a plasma is sustained between a grounded electrode 106 and a powered lower electrode (chuck) 105. A first RF generator 134 generates the plasma as well as controls the plasma density, while a second RF generator 138 generates bias RF, commonly used to control the DC bias and the ion bombardment energy.
Further coupled to source RF generator 134 and bias RF generator 138 is matching network 136 that attempts to match the impedances of the RF power sources to that of plasma 110. Furthermore, pump 111 is commonly used to evacuate the ambient atmosphere from plasma chamber 102 in order to achieve the required pressure to sustain plasma 110. In addition, plasma 110 may be confined between chuck 105 and grounded electrode 106 by means of confinement rings 103, which may control a pressure within plasma 110. Confinement rings 103 can be moved to increase and decrease a spacing or gap between adjacent confinement rings, commonly by the use of cam ring. Gas distribution system 122 is commonly comprised of compressed gas cylinders containing plasma processing gases (e.g., C4F8, C4F6, CHF3, CH2F3, CF4, HBr, CH3F, C2F4, N2, O2, Ar, Xe, He, H2, NH3, SF6, BCl3, Cl2, WF6, etc).
In general, in order to achieve a substantially uniform enchant gas distribution across the surface of a substrate, the grounded electrode typically includes perforations or pores, in a showerhead configuration, through which the plasma gases may pass into the plasma chamber. In a common configuration, an electrode assembly usually includes the chamber lid (in order to securely attach the electrode components in the plasma chamber), a cooling plate and a heating plate (in order to prevent plasma gas reactions from taking place within the perforations or pores), a backing plate (in order to electrically isolate the electrode from the heating plate and cooling plate, while still allowing a thermal path between the heating plate and cooling plate and the grounded electrode), and the grounded electrode itself (in order to distribute the plasma gas across the surface of the substrate, as well as to provide a rf return ground path for the powered electrode).
Referring now to FIG. 2, a simplified diagram of a common electrode assembly configuration is shown. Chamber lid 212 is generally configured to mate with the plasma chamber in order to maintain a substantial vacuum for plasma processing. In general, chamber lid 212 comprises a top plate 212b and circular stub 212a that protrudes into the plasma chamber (not shown) and provides a planar surface to which a cooling plate 208 or heating plate 206 may be attached. Circular stub 212a is further surrounded by a groove (defined by circular stub 212a and top plate 212b) into which gasket 214 is commonly placed. In one configuration, an electrode sub-assembly (including in sequence a cooling plate 208, a heating plate 206, a backing plate 204, and grounded electrode 202) is assembled or sandwiched in a unitary construction, with a first set of metal fasteners, and then attached as a unit to circular stub 212a with a second set of metal fasteners, in order to assemble the electrode assembly.
Cooling plate 208 may be cooled by a chiller system that re-circulates fluid through cavities in within cooling plate 208. In addition, the fluid can be a liquid (e.g., water, etc.) or a gas (e.g., air, etc.). The liquid or air can be chilled for greater cooling effect and can be re-circulated for greater efficiency. This fluid is, in turn, commonly pumped through a set of conduits to an external source of heat convection, such as a heat exchanger, and returned back to the chuck. Heating plate 206 generally includes a set of resistive elements that output thermal energy to when the set of elements is supplied with electrical current. In combination with cooling plate 208, heating plate 206 allows the plasma gas temperature to be sustained within recipe parameters in order to generally maintain etch quality and substrate yield. Backing plate 204 is commonly made of graphite, backing plate generally provides temperature uniformity across grounded electrode 202.
However, backing plate 204 is generally made of a material that is relatively soft (e.g., graphite, etc.). Consequently, helicoil inserts are often required in order to properly mate with threaded metal fasteners. A helicoil is generally an internal thread insert for creating stronger threads in any assembly prone to thread damage. However, the use of different materials with differing thermal expansion rates may cause defects to form in the electrode assembly as it is repeatedly heated and cooled. The coefficient of thermal expansion (α) is generally defined as the fractional increase in length per unit rise in temperature. The exact definition varies, depending on whether it is specified at a precise temperature (true α) or over a temperature ranges (mean α). The former may be related to the slope of the tangent to the length—temperature plot, while the latter may be governed by the slope of the chord between two points on this curve.
In general, the metal portions of the electrode assembly (e.g., cooling plate 208, heater plate 206, threaded bolts, etc.) generally have a higher α than the non-metal portions of the electrode assembly (e.g., backing plate, etc.). For example, aluminum (commonly used in cooling plate 208, heating plate 206, grounded electrode 202, metal fasteners, etc.) generally has a relatively large α (e.g., 23.1×10−6 K−1), in comparison to graphite (commonly used in backing plate 204) with a smaller α (e.g., 6.5×10−6 K−1). That is, per a unit increase in temperature, aluminum may expand up to four times as much as graphite. However, the electrode assembly is assembled in a unitary construction with metal fasteners that extend through the various components (e.g., cooling plate, heater plate, backing plate, grounded electrode, etc.), and thus has minimal lateral and longitudinal play between the components. Consequently, repetitive cycling of temperature may stress and hence damage the backing plate 204, and consequently produce graphite particles that may contaminate the plasma chamber. In general, the lateral axis is parallel to the substrate surface, whereas the longitudinal axis is perpendicular to the substrate surface.
In addition, the use of metal fasteners in the electrode sub-assembly may also increase the likelihood of arcing. An arc is generally a high power density short circuit which has the effect of a miniature explosion. When arcs occur on or near the surfaces of the target material or chamber fixtures, substantial damage can occur, such as local melting. Plasma arcs are generally caused by low plasma impedance which results in a steadily increasing current flow. If the resistance is low enough, the current will increase indefinitely (limited only by the power supply and impedance), creating a short circuit in which all energy transfer takes place. This may result in damage to the substrate as well as the plasma chamber. For example, as the rf electrical charge is drained away from the powered electrode toward the grounded electrode, a secondary electrical discharge may occur with a metal fastener, particularly across a perforation or pore.
Furthermore, the large number of metal fasteners also makes assembling, aligning, replacing, and/or installing the electrode sub-assembly components problematic. For example, fastener tolerances may be relatively tight (e.g. 1/1000th inch, etc.). However, after the electrode assembly has been repeatedly exposed to plasma and/or temperature cycling, the actual tolerance may have been reduced (tolerance shrinkage). For example, contaminants may have wedged themselves into the helicoil, or the fastener holes may have decreased in size, etc. Consequently, the electrode assembly may be difficult to remove in a safe manner, without damage occurring to the electrode assembly itself, and without the use of multiple tools (e.g., screwdriver, hammer, wedge, etc.). For example, graphite is a relatively brittle material. However, if a hammer needs to be used to dislodge the cooling plate or heating plate from the backing plate, substantial damage may occur to the graphite.
In addition, since most common electrode assembly configurations do not generally include a dedicated rf return path, the electrical characteristics of ground may change between successive substrates or alternate recipes, depending on electrode assembly wear, or the electrical characteristics of the plasma gases used. Generally, most materials used in the electrode assembly are electrically conductive. However, the exact return path to ground may physically shift, and hence may affect the electrical load on the plasma. For a given process recipe, it is generally beneficial for the rf power delivery to remain stable throughout the plasma process in order to obtain a reliable process result.
In view of the foregoing, there are desired methods and apparatus for an optimized plasma chamber electrode assembly.